refactor(tvix): move tvix glue code into glue crate r/6936

There's various bits and pieces in tvix-cli that use both the store and
evaluator, as well as nix-compat. For example, builtins.derivation, as
well as the reference scanning implementation.

This "glue code" currently isn't accessible from anywhere else, but it'd
be very useful if it were.

Move it out into a `glue` crate, and make `tvix-cli` a consumer of it.

All the KnownPaths setup and passing around, as well as NIX_PATH
handling is also something that should probably be moved into the glue
crate as well, but that's something left for a future CL.

Change-Id: I080ed3d1825ab23790666486840f301f00856277
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9908
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

1 files changed, 186 insertions, 0 deletions

diff --git a/tvix/glue/src/known_paths.rs b/tvix/glue/src/known_paths.rs
new file mode 100644
index 000000000000..07373ef0da7a
--- /dev/null
+++ b/tvix/glue/src/known_paths.rs
@@ -0,0 +1,186 @@
+//! This module implements logic required for persisting known paths
+//! during an evaluation.
+//!
+//! Tvix needs to be able to keep track of each Nix store path that it
+//! knows about during the scope of a single evaluation and its
+//! related builds.
+//!
+//! This data is required to scan derivation inputs for the build
+//! references (the "build closure") that they make use of.
+//!
+//! Please see //tvix/eval/docs/build-references.md for more
+//! information.
+
+use crate::refscan::{ReferenceScanner, STORE_PATH_LEN};
+use nix_compat::nixhash::NixHash;
+use std::{
+    collections::{hash_map, BTreeSet, HashMap},
+    ops::Index,
+};
+
+#[derive(Debug, PartialEq)]
+pub enum PathKind {
+    /// A literal derivation (`.drv`-file), and the *names* of its outputs.
+    Derivation { output_names: BTreeSet<String> },
+
+    /// An output of a derivation, its name, and the path of its derivation.
+    Output { name: String, derivation: String },
+
+    /// A plain store path (e.g. source files copied to the store).
+    Plain,
+}
+
+#[derive(Debug, PartialEq)]
+pub struct KnownPath {
+    pub path: String,
+    pub kind: PathKind,
+}
+
+impl KnownPath {
+    fn new(path: String, kind: PathKind) -> Self {
+        KnownPath { path, kind }
+    }
+}
+
+/// Internal struct to prevent accidental leaks of the truncated path
+/// names.
+#[repr(transparent)]
+#[derive(Clone, Debug, Default, PartialEq, PartialOrd, Ord, Eq, Hash)]
+pub struct PathName(String);
+
+impl From<&str> for PathName {
+    fn from(s: &str) -> Self {
+        PathName(s[..STORE_PATH_LEN].to_string())
+    }
+}
+
+/// This instance is required to pass PathName instances as needles to
+/// the reference scanner.
+impl AsRef<[u8]> for PathName {
+    fn as_ref(&self) -> &[u8] {
+        self.0.as_ref()
+    }
+}
+
+#[derive(Debug, Default)]
+pub struct KnownPaths {
+    /// All known paths, keyed by a truncated version of their store
+    /// path used for reference scanning.
+    paths: HashMap<PathName, KnownPath>,
+
+    /// All known derivation or FOD hashes.
+    ///
+    /// Keys are derivation paths, values is the NixHash.
+    derivation_or_fod_hashes: HashMap<String, NixHash>,
+}
+
+impl Index<&PathName> for KnownPaths {
+    type Output = KnownPath;
+
+    fn index(&self, index: &PathName) -> &Self::Output {
+        &self.paths[index]
+    }
+}
+
+impl KnownPaths {
+    fn insert_path(&mut self, path: String, path_kind: PathKind) {
+        match self.paths.entry(path.as_str().into()) {
+            hash_map::Entry::Vacant(entry) => {
+                entry.insert(KnownPath::new(path, path_kind));
+            }
+
+            hash_map::Entry::Occupied(mut entry) => {
+                match (path_kind, &mut entry.get_mut().kind) {
+                    // These variant combinations require no "merging action".
+                    (PathKind::Plain, PathKind::Plain) => (),
+                    (PathKind::Output { .. }, PathKind::Output { .. }) => (),
+
+                    (
+                        PathKind::Derivation { output_names: new },
+                        PathKind::Derivation {
+                            output_names: ref mut old,
+                        },
+                    ) => {
+                        old.extend(new);
+                    }
+
+                    _ => panic!(
+                        "path '{}' inserted twice with different types",
+                        entry.key().0
+                    ),
+                };
+            }
+        };
+    }
+
+    /// Mark a plain path as known.
+    pub fn plain<S: ToString>(&mut self, path: S) {
+        self.insert_path(path.to_string(), PathKind::Plain);
+    }
+
+    /// Mark a derivation as known.
+    pub fn drv<P: ToString, O: ToString>(&mut self, path: P, outputs: &[O]) {
+        self.insert_path(
+            path.to_string(),
+            PathKind::Derivation {
+                output_names: outputs.iter().map(ToString::to_string).collect(),
+            },
+        );
+    }
+
+    /// Mark a derivation output path as known.
+    pub fn output<P: ToString, N: ToString, D: ToString>(
+        &mut self,
+        output_path: P,
+        name: N,
+        drv_path: D,
+    ) {
+        self.insert_path(
+            output_path.to_string(),
+            PathKind::Output {
+                name: name.to_string(),
+                derivation: drv_path.to_string(),
+            },
+        );
+    }
+
+    /// Checks whether there are any known paths. If not, a reference
+    /// scanner can not be created.
+    pub fn is_empty(&self) -> bool {
+        self.paths.is_empty()
+    }
+
+    /// Create a reference scanner from the current set of known paths.
+    pub fn reference_scanner(&self) -> ReferenceScanner<PathName> {
+        let candidates = self.paths.keys().map(Clone::clone).collect();
+        ReferenceScanner::new(candidates)
+    }
+
+    /// Fetch the opaque "hash derivation modulo" for a given derivation path.
+    pub fn get_hash_derivation_modulo(&self, drv_path: &str) -> NixHash {
+        // TODO: we rely on an invariant that things *should* have
+        // been calculated if we get this far.
+        self.derivation_or_fod_hashes[drv_path].clone()
+    }
+
+    pub fn add_hash_derivation_modulo<D: ToString>(
+        &mut self,
+        drv: D,
+        hash_derivation_modulo: &NixHash,
+    ) {
+        #[allow(unused_variables)] // assertions on this only compiled in debug builds
+        let old = self
+            .derivation_or_fod_hashes
+            .insert(drv.to_string(), hash_derivation_modulo.to_owned());
+
+        #[cfg(debug_assertions)]
+        {
+            if let Some(old) = old {
+                debug_assert!(
+                    old == *hash_derivation_modulo,
+                    "hash derivation modulo for a given derivation should always be calculated the same"
+                );
+            }
+        }
+    }
+}